1. Field of the Invention
The present invention relates to a motor driver for driving a polyphase motor by applying a pulse-width-modulated voltage to a coil terminal of the motor to supply a drive current to the motor coil.
2. Discussion of the Related Art
In the field of household electrical appliance, office automation equipment and electric motor drive for driving vehicle, commutatorless motors commutating with an electric circuit using a switching element such as brushless motors and induction motors are widely used. Japanese published unexamined application No. 7-241095 discloses a method of applying a voltage having a predetermined phase difference to each coil terminal according to a basic waveform, an electric angle and a voltage magnitude by setting the basic waveform of the applied voltage such that the applied voltage to a phase is constantly zero for the purpose of reducing the applied voltage peak in the drive circuit and the switching loss. Further, when controlling speed by vector control method, the method disclosed in Japanese published unexamined application No. 7-241095 cannot be used because the applied voltage of each phase does not have a fixed phase difference. However, Japanese published unexamined application No. 2006-129663 discloses a method having an effect equivalent to that of Japanese published unexamined application No. 7-241095 by selectively switching a phase pausing switching with a phase of the applied voltage vector in a fixed coordinate system of orthogonal two axes, obtained from a control arithmetic and a axis transformation.
The method of Japanese published unexamined application No. 7-241095 applying a voltage having a predetermined phase difference among phases in accordance with a basic waveform cannot be used when the voltage applied to each phase does not have a phase difference.
As Japanese published unexamined application No. 2006-129663, when forming a speed control loop by controlling vector, plural multiplications and size comparison are needed to determine a phase to pause switching against a phase of the applied voltage vector in a fixed coordinate system of orthogonal two axes and separate the vector. Therefore, it takes a long time to compute when realizing a modulator on a processor with a software and a circuit size is large when realizing with a digital circuit. As for the control method of having no predetermined phase difference among each phase and not computing an applied voltage vector in the process of control computation, a biaxial applied voltage vector is synthesized from an applied voltage value and a direction vector of each phase and a phase to pause switching is determined, resulting in a long compute time or a large circuit size.
Depending on the modulation method of the applied voltage, the electric power differs before and after the modulation, resulting in no modulation and different motor behavior.
Since the applied voltage modulation varies the applied voltage waveform, there is a section a current cannot be run through depending on a drive status of the switching element and a direction of the coil current.
Because of these reasons, a need exists for a motor drive circuit capable of lowering applied voltage peak and reducing switching loss without increasing compute time and digital circuit regardless of phase difference.